A deletion of the long arm of chromosome 20 [del(20q)] is a recurring abnormality in malignant myeloid diseases. By combining molecular analysis to detect allele loss with fluorescence in situ hybridization (FISH) of probes from 20q, we have delineated the smallest commonly deleted segment in myeloid disorders characterized by a del(20q). We hypothesize that loss of function of a tumor suppressor gene located within the commonly deleted segment plays a role in the pathogenesis of these disorders. We now propose complementary approaches to isolate the putative tumor suppressor gene. The studies utilize cytogenetic and molecular approaches to narrow the commonly deleted segment in 20q12 by using FISH analysis of a panel of patients with (1) interstitial deletions of 20q, (2) unbalanced translocations leading to loss of 20q, and (3) balanced translocations with breakpoints in 20q11.2-13.1 (Aim 1). To identify candidate genes, we will prepare a transcript map of the commonly deleted segment by mapping known genes and expressed sequence tags within the region, and by searching for previously unidentified expressed sequences (Aim 2). To identify the gene whose altered function plays a role in leukemogenesis, candidate genes will be examined for mutations in myeloid leukemia cells. If mutations of a gene on 20q are identified, we will determine the spectrum of mutations involving the gene in myeloid leukemia cells with abnormalities of 20q (Aim 3). In the final aim, we will characterize the myeloid leukemia gene and protein, identify the consequence(s) of the mutations on the function of the gene/protein, and generate and characterize a mouse model that is null for this putative tumor suppressor gene (Aim 4). Tumor suppressor genes have not been well-characterized in leukemias. Nevertheless, the high frequency of recurring chromosomal deletions in myeloid leukemias suggests that tumor suppressor genes may play a role in the pathogenesis of these disorders. By examining the del(20q), we can begin to evaluate the role of recessive mutations in the pathogenesis of malignant myeloid diseases as well as identify a gene that is involved in normal hematopoiesis, and in leukemogenesis.